1. Field of the Invention
A present invention relates to a pyroelectric thermal sensor having an extended temperature measuring range.
The pyroelectric thermal sensor of the present invention is operative to extend the temperature measuring range, which has been limited due to the saturation voltage of the amplifier, without switching the amplifier, the sensor being useful for the temperature control of the fixing heat rollers for electrophotography.
2. Prior Art
FIG. 13 is a block diagram explaining the conventional pyroelectric thermal sensor, and FIG. 14 is a waveform diagram showing the output signals at various portions of FIG. 13.
As shown in the figure, the pyroelectric thermal sensor includes a heat ray interrupting means 1 controlled by driving controller 20 which interrupts the heat ray 90 radiated from an object 9 under temperature measurement, a pyroelectric element 3 disposed in the rear of the interrupting means 1 and operates to detect the difference between its periphery temperature and the surface temperature of the object 9, and a reference thermal sensor 6 which detects the temperature in the periphery of the pyroelectric element 3.
The outputs of the pyroelectric element 3 and the reference thermal sensor 6 are amplified by the amplifiers 4 and 60, respectively, to produce the pyroelectric element amplified output (B) and reference thermal sensor amplified output (C), and these output signals are composed by the adding circuit 80 to produce the summed output (D) representing the surface temperature of the object 9. In other words, the surface temperature of the object 9 is given as a sum of the outputs of the pyroelectric element 3 and the reference thermal sensor 6. The composed temperature signal is processed by a microcomputer or the like, and then used for controlling the temperature indicator 91, the heater 92 or the like.
FIG. 15 shows a commonly used amplifier, and FIG. 16 explains the output level of the amplifier. The illustrated amplifier has its output Vo given as: EQU Vo=-(Rf/Ri)Vin (1)
Namely, the gain of the amplifier can be adjusted by the value of the feedback resistor Rf.
The output level of the amplifier is saturated at its driving voltage VDD as shown in FIG. 16. On this account, the gain of the amplifier is generally set so that a most frequent occurring output level Vfr is about VDD/2.
In the following, it is intended to apply this idea to the pyroelectric thermal sensor. For example, when a pyroelectric thermal sensor is used for controlling the temperature of the electrophotographic fixing heat roller, the controlled temperature is generally around 200.degree. C. A pyroelectric element used in the pyroelectric thermal sensor has its output in the function of the fourth power of the detected temperature, and therefore the pyroelectric element produces a very small output signal in the low-temperature range. On this account, if the amplifier gain is set so that the output V200 for 200.degree. C. (V200 is equivalent to the Vfr) is equal to VDD/2, the measurable temperature range will be 100.about.230.degree. C. and it is difficult to measure a temperature below 100.degree. C.
In the operation of the heat roller, if the temperature in the above-mentioned low temperature range cannot be detected, checking for the abnormalities of the heater and the sensor will be deterred in the initial phase, resulting possibly in a damage to the heat roller or the sensor.
Conceivable methods for overcoming the above problem include selective use of amplifier and change of the amplifier gain. These methods, however, requires a complicated circuit structure and therefore results in a degraded reliability.